1. Field of the Invention
The present invention relates to an image forming apparatus that writes an electrostatic latent image on a photosensitive member by using a rotatable polygonal mirror (polygon scanner), and, in particular, to a reduction in first print time thereof.
2. Description of the Related Art
It is desired for image forming apparatuses, such as a copier and a printer, that an image be formed on different kinds of sheets at high speeds, with high quality, and with high precision. In many cases, an image forming apparatus that aims to enhance the speed uses, as an optical scanning device (optical system), a polygonal mirror drive motor for driving a laser beam and a rotatable polygonal mirror for deflecting the lager beam.
In such an image forming apparatus, when a start signal that indicates the start of a copying operation is input by entry of a copy start key from a user or of a print instruction from a computer, an initiation operation (e.g., high-pressure activation) used for an electrophotography process (e.g., charging or transferring) starts. In synchronism therewith, a polygonal mirror drive motor is activated from a stopped state or a low-speed rotation state to a high-speed rotation state required for image formation (i.e., the speed is shifted). After the polygonal mirror drive motor reaches a predetermined rotation speed, an image forming operation is performed. Such a sequence is typically used.
Since the polygonal mirror drive motor has a large interior force because it rotates a polygonal mirror, a long time is necessary for reaching a stable rotation state (activated state). In general, a time of 2 to 7 seconds is needed for rising from a stopped state or a low-speed rotation state to a high-speed rotation state. For this reason, a reduction in the time required for activation of the polygonal mirror drive motor is considered as one of major techniques for reducing a first copy time, i.e., a time from when the copy start button is pressed down to when a first sheet is ejected. Japanese Patent Laid-Open No. 7-273951 discloses an image forming apparatus that has a function of previously activating a polygonal mirror drive motor when pressing of a key of an operation portion other than a copy start key by a user is detected. Japanese Patent Laid-Open No. 2002-326386 discloses another image forming apparatus that has a plurality of process speeds and that allows an operator to select the speed of the previously rotated polygonal mirror drive motor.
An image forming apparatus that has a plurality of process speeds, for example, that has two image forming modes, i.e., monochrome and color modes, or that can switch the resolution of an image to be formed, has a plurality of stages for the number of revolutions (rotation speed) of a polygonal mirror drive motor. Generally, such an image forming apparatus determines information about the image forming mode or about the resolution with approximately the same timing as in reception of image information. Therefore, if a function of previously activating the polygonal mirror drive motor is performed, it is necessary to activate the polygonal mirror drive motor at any one of the plurality of stages for the rotation speed.
For an arrangement in which the rotation speed for the start of the polygonal mirror drive motor is selected or preset to a fixed value, if the selected rotation speed is inappropriate, a time for shifting to an appropriate rotation speed is necessary. This results in an undesirable increase in the first copy time (first print time).